PROJECT SUMMARY Meiosis is a specialized cell division program that gives rise to gametes in sexually reproducing organisms. The first stage of meiosis, called meiosis I, uniquely involves the association, programmed recombination, and eventual segregation of homologous chromosomes. While this process is well understood from a genetic and cytological standpoint, our understanding of how the meiosis-specific cellular machinery is able to organize and manipulate meiotic chromosomes in 3D space to mediate their proper segregation remains a mystery. This area of study is significant, as errors in meiosis I chromosome segregation account for the vast majority of aneuploidies, extra or missing chromosomes in offspring, that occur in over half of human oocytes and 5-10% of clinically recognized pregnancies. As such, aneuploidy is the leading genetic cause of miscarriage and of mental retardation (e.g. Down syndrome, caused by trisomy of chromosome 21). The underlying causes of chromosome segregation errors in meiosis I are not well understood, and further progress toward identifying these causes will require a detailed understanding of the molecular mechanisms of meiosis-specific chromosome segregation machinery. Here, we propose to study three sets of meiotic chromosome-associated proteins that are critical for different aspects of chromosomes' organization and physical manipulation in meiosis I. Our approach combines in vitro reconstitution of purified proteins and complexes, 3D structural analysis of these complexes, and targeted genetic assays to test mutants designed to disrupt specific aspects of these proteins' structures and interactions. We will first study the S. cerevisiae monopolin complex, which binds chromosomes' kinetochores in meiosis I and modifies their attachments to spindle microtubules, to enable the proper orientation and segregation of homologous chromosomes. We will determine the architecture of the monopolin complex, and use engineered protein constructs in genetic assays to test whether it directly cross-links sister kinetochores to mediate their attachment to a single microtubule. Next, we will study the conserved chromosome-associated protein Hop1, a component of the proteinaceous axis about which each chromosome is organized. We will examine the roles of Hop1's conserved HORMA domain, a signaling domain shared with the spindle checkpoint protein Mad2, in regulating inter-homolog meiotic recombination, and in a meiosis-specific checkpoint monitoring recombination. Finally, we will study the synaptonemal complex, an essential polymeric assembly that links homologs together during meiotic recombination. As very little is known about the architecture of this complex or its functions, we will study the domain structure, protein-protein interactions, and self-assembly determinants of the key S. cerevisiae synaptonemal complex protein Zip1. Combined, this work will begin to provide a more accurate picture of the macromolecular structures and interactions underlying homologous chromosome recombination and segregation in meiosis I.